Create task to export translations for our Categories

Currently Banyan Categories and Groups pull the text from the corresponding translation tables.
We need to export the translation tables into a file that can be passed to translators.
The format of the file must be human readable and contain information that would easily allow to re-import data from this file back into the system. Key value pair similar to yml would be one choice. 

We need one file for category groups and one for categories.

Please attache the files to this story and send the copies to Jacek.


Instructions for Implementers
===

```gherkin
Feature: Facilitating translation of Banyan Categories

  Scenario: Exporting Banyan Category Translations
    When I run `rake translations:categories:export[current_translations.yml]`
    Then the file "current_translations.yml" should contain "en:"
    And the file "current_translations.yml" should be valid yaml

  Scenario: Importing Banyan Category Translations
    Given a file named "new_translations.yml" with:
    """
    ---
    de:
      banyan_category_translations:
        Business: Unternehmen
        "Tools & Utilities": Werkzeuge und Hilfsprogramme
      banyan_category_group_translations:
        appzone: AppZone
        categories: Kategorien
        form_factors: Formfaktoren
    """
    When I run `rake translations:categories:import[new_translations.yml]`
    Then the output should contain "Added new locale (de) with 5 translations"

  Scenario: Import and Export
    Given a file named "new_translations.yml" with:
    """
    ---
    de:
      banyan_category_translations:
        Business: Unternehmen
        "Tools & Utilities": Werkzeuge und Hilfsprogramme
      banyan_category_group_translations:
        appzone: AppZone
        categories: Kategorien
        form_factors: Formfaktoren
    """
    And I run `rake translations:categories:import[new_translations.yml]`
    When I run `rake translations:categories:export[current_translations.yml]`
    Then the file "current_translations.yml" should contain "form_factors: Formfaktoren"
```

Further details
===

1) Export task YAML output structure should match Import task YAML input structure:
a) Top level keys matching locales, IE: each of "en", "de", "fr" are sibling nodes.
b) Two child keys for each locale, banyan_category_translations and banyan_category_group_translations
c) use the "tag" property of the Banyan::Category or Banyan::CategoryGroup as the key for the translations nested under the banyan_category_translations or banyan_category_group_translations sections

2) Import task must perform operation in a single transaction

3) Export task must consolidate duplicate category group translations, to avoid repetition
4) Import task must expand duplicate category group translations, mirroring the structure of the english locale
- It should not change the Banyan::CategoryGroup ID, the number of Banyan::CategoryGroups must remain the same before and after the import, only the number of translations should change.

5) Make these changes to the Banyan gem. Load the rake task from the banyan gem into the rails application
6) The cucumber tests should be in the Banyan gem.
7) The rake tasks should just use simple ruby objects, so that we can put the functionality under unit test with low coupling